Some varieties of flexible printed circuitry are prepared by etching selected areas of metal from a metal-clad nonwoven fibrous web. However existing metal-clad nonwoven fibrous webs have been inadequate for use in certain kinds of printed circuitry that require the web to exhibit an extreme degree of dimensional stability during manufacturing and subsequent processing operations of the printed circuitry. For example, to be useful in those kinds of printed circuitry, the nonwoven fibrous web should shrink no more than 10 mils/inch of length during such manufacturing and processing operations, which include etching operations in which metal is removed in areas, and soldering operations in which the printed circuitry is briefly contacted with a molten bath of solder heated to 500.degree. F or more. If the web shrinks more than 10 mils/inch of length, particular sections of the printed circuitry will be out of alignment with electrical components or other circuitry that is to be connected to the printed circuitry. The need for dimensional stability has increased because of an increasing need for larger sheets of printed circuitry, in which changes in dimension accumulate to large overall changes in dimension.
Other necessary properties of the nonwoven fibrous web include good toughness and tear strength (a tear strength of at least 4 pounds (Graves Tear Test as described in ASTM D-1004) or more is desirable); ability to be cut cleanly without formation of projecting fibers; and low capacity for absorbing moisture, which tends to cause blistering of the metal layer away from the nonwoven fibrous web when the metal-clad sheeting is heated on a solder bath.
One existing metal-clad nonwoven fibrous web that generally has quite useful properties for printed circuitry, but which does not always exhibit the extreme degree of dimensional stability described above, is taught in Groff, U.S. Pat. No. 3,878,316. The nonwoven fibrous web that is the backing for that metal-clad product comprises a blend of fibers including heat-softenable fibers that are heat-softened during the process of manufacture of the web to bind the fibers of the nonwoven web into an integral whole (an exemplary fiber blend described in the application comprises 50 weight-percent undrawn heat-softenable polyester fibers, 25 weight-percent drawn polyester fibers, and 25 weight-percent high-temperature resistant aromatic polyamide fibers). Despite the many useful properties of a nonwoven fibrous web as described in that application, such webs have not satisfied the need for webs having the extreme dimensional stability described above. Nor has any other known prior-art nonwoven fibrous web satisfied that need.